Field of the Invention
The present invention relates to spectral color sensors and image forming apparatuses such as printers and photocopiers, and particularly relates to the removal of stray light components in a spectral color sensor.
Description of the Related Art
Conventionally, color conversion lookup tables (abbreviated as “LUT” hereinafter) are used in printing apparatuses such as printers in order to output desired colors. Color conversion LUTs include LUTs used in calibration for keeping the printer in a certain stable state, LUTs used in color matching such as ICC profiles, and so on.
Some models of printers include a spectral color sensor within the printer engine. This type of printer prints a color index (called a “patch” hereinafter) such as IT8 7/3 (ISO 12642 output target) prior to or during the execution of a print job, and measures the patch using the included spectral color sensor. The color measurement results are then fed back in order to generate a color conversion LUT. Through this, the stated printer can match colors and stabilize printing colors through a process internal to the printer, rather than using an external color measurement sensor.
Such a spectral color sensor includes light sources such as a white LED, a halogen lamp, three RGB LEDs, and so on within the apparatus, and irradiates a color measurement target with color measurement light. The spectral color sensor then constricts the reflected light therefrom using a slit, disperses the light using a prism, a diffraction grating, or the like, and obtains a spectral distribution. Area sensors are often used as the light detection elements in such spectral color sensors in order to reduce the size of the sensor unit, reduce the measurement time for each patch, and so on. However, it is known that a high color measurement accuracy cannot be obtained if the relationship between the pixels in the area sensor and the wavelength of the reflected light is not precise. For example, in the case where a spectral color sensor is provided within a printer, the optical system in the sensor will change as a result of the sensor body warping due to rising temperatures in the printer, differences in installation environments, the passage of time, and so on, thermal expansion of the diffraction grating, or the like. As a result, there are cases where the pixel-to-wavelength relationship of the area sensor shifts from the relationship that was certified at the time of assembly.
In response to this problem, as a known technique, a method has been proposed in which the wavelengths of the light entering into the pixels are specified by overlaying on each pixel of the area sensor a color filter that allows only visible light wavelengths to pass to the pixels of the area sensor. Alternatively, a LED single-color light source for wavelength calibration is provided within the spectral color sensor, and the pixel-to-wavelength relationship of the area sensor is recertified based on the absolute value of the emission line of the LED single-color light source, as in Japanese Patent Laid-Open No. 2008-185565.
Incidentally, the actual output of the spectral color sensor is the sum of the signal from the measurement target that is originally desired, and noise. In addition to dark outputs from light-receiving elements, stray light components that cannot be completely removed are present in noise. If these stray light components are not taken into consideration, they become noise components in the original signal, and cause a drop in the color measurement accuracy. Dark areas in particular have a low reflectance and a high influence on the color measurement accuracy, and thus it is necessary to remove such areas accurately. A method for removing stray light components when an optical sensor is in use is known, as disclosed in, for example, Japanese Patent Laid-Open No. 6-22134. Note that stray light is light resulting from some of the light source leaking within the sensor unit and that reaches the light-receiving element, causing noise. For the amount of stray light, it is normally necessary to provide a dedicated light absorbing body, such as a black body that absorbs irradiated light, on the color measurement surface that is irradiated with light when the light source is lighted, and measure only the stray light components.
It is assumed that the stray light components will fluctuate over periods of time such as hours or years, and thus it is desirable to be able to take measurements (update) as necessary. However, in the case where an attempt is made to measure stray light components with a printer that includes a spectral color sensor, a configuration that allows measurement using a white plate and a dedicated light absorbing body in an alternating manner is required, and there have thus been problems in that the space required for installation has increased, the apparatus has become complicated, and so on. Furthermore, even if the printer is not a type that, for example, is provided with an internal spectral color sensor, from the standpoint of usability, it is undesirable to employ a separate apparatus.